Tropical Fish Are Heading for the Poles

When marine ecologist Adriana Vergés emerged from a dive in southern Japan's Tosa Bay, she was amazed at what she saw: once-lush kelp forest been stripped bare and replaced by coral. Tosa Bay is hundreds of miles north of the tropics, but now "it feels like a tropical place," said Vergés.

The undersea world is on the move. Climate change is propelling ocean life into what used to be cooler waters, and researchers are finding that the repercussions of tropical fish migration are often devastating. Invading tropical species are stripping kelp forests in Japan, Australia, and the eastern Mediterranean, and chowing down on sea grass in the northern Gulf of Mexico and Atlantic seaboard. "The faunas are mixing, and nobody can see what the outcome will be," said Ken Heck, a marine scientist at the University of South Alabama. But the consequences of that mixing are already trickling up the food chain.

Sea grass beds and kelp forests are the sea's nurseries, because they feed and protect fish larvae and juveniles. But they are being replaced with other warm-water species such as coral that follow the arrival of tropical fish. In a study for the Proceedings of the Royal Society B in July, Vergés and her colleagues note that many species clean coral of algae and plants that could otherwise choke the reefs. But when the fish move toward more temperate waters, they often find a bountiful harvest of kelp or sea grass to feed their voracious appetites. Shrimp, crab, and other species that often spend the first year of their lives hiding from predators in the protection of the grasses disappear when their cover is gone, leaving a void for creatures that depend on them. It also has devastating effects on commercial fisheries. In southern Japan, the arrival of rabbitfish and parrotfish destroyed as much as 40 percent of kelp forests there, which were once thick with abalone and spiny lobster, and supported a famed fishery.

In the northern Gulf of Mexico, Heck started seeing unusual species back in 2006 and 2007 near the Dauphin Island Sea Lab, south of Mobile, AL. Comparing the numbers of tropical fish in his team's trawl nets with records from the 1970s, Heck found a 22-fold increase in emerald parrotfish -- a coral cleaner common in the Caribbean -- and new arrivals of snapper, grouper, butterflyfish and surgeonfish. Green sea turtles and manatees have also found northern Gulf waters increasingly comfortable.

Angelfish and damselfish have been found off the Carolinas, and the Chesapeake's blue crabs have been seen as far north as New York. Plants are also on the march: Black mangroves are growing miles farther north along the Florida coast than they were a decade ago. Species on land and in the oceans have always moved around, but now climate change is increasing the speed of this movement, Vergés said. "There's no doubt. The magnitude of the change is so large that it's very obvious."

- - Doug Struck, National Geographic News